Marine risers are integral part of drilling and production activity of oil and gas exploration. They provide means for drill strings and production tubing to reach the oil well deep down at the ocean floor. Several buoyant structures (e.g. platforms, ships) are often used to support oil and gas processing equipment, storage facilities or other facilities where fluids which are being transferred between the seabed and the buoyant structure. Marine risers comprise sections of pipes sometimes called “conductors” or “casings” connected together as a “string” of “risers” and the vessels are suitably outfitted “ships” or “semi-submersibles”, also referred to as “rigs”.
In shallow water (e.g. less than 100 meters), different types of risers are available for FPSO (Floating Production, Storage and Offloading) vessels that have been developed for operations in shallow water. One such riser type comprise unbonded steel flexible pipe, having steel layers wound around an inner carcass, covered with a plastic sheathing. Such flexible risers are important components for offshore developments because they can accommodate the large motions induced by floating structures and can also resist hydrodynamic loadings such as waves and currents. The flexible risers have high axial stiffness and low bending stiffness. These properties increase the ability of the flexible riser to handle large deformations. These large deformations may be generated by ocean currents and/or waves, or by the motions of the floating structure. Other types of flexible risers are bonded pipes (hoses) and composite pipes.
One of the main characteristics of riser systems in shallow water is the large degree of compliancy required to accommodate the relatively large vessel offsets compared to water depth. Several attempts have been made in order to develop riser configure that offer a large degree of compliancy.
A variety of configurations are used when suspending the riser between the floating structure and the seabed. A considerable part of a flexible riser system is the determination of the configuration so that the riser can safely sustain the extreme seastates loading. In general, the critical sections in the riser configurations are at the ends (top or bottom), where there are high tensile forces and large curvatures. They are also critical at the sag bend, where there is large curvature (at low tension), and at the hog of a wave buoyancy section, where there is large curvature (at low tension). The standard riser configurations generally used in the industry are “free-hanging catenary”, “lazy-S”, “Lazy wave”, “Steep-S”, “compliant wave” and “steep wave”. These different riser configurations are obtained by use of various configurations of buoyancy elements, weight elements and tether lines that are fixed to the riser and sometimes anchored to the seabed.
Traditional riser systems for shallow water are very little offset-friendly, leading to strict offset tolerances and heavy mooring systems. Normally, the traditional riser systems have a so-called two-dimensional (2D) wave configuration where buoyancy modules, acting as springs, bring the riser to a wave configuration in the vertical (x-z) plane. This riser configuration is not suitable for shallow water, especially not for waters shallower than 40 meters. In these environments, the riser may be subject to excessive bending, overstretching and interference with the floating structure, mooring devices, buoyancy devices, and the seabed.
The prior art includes EP 0 729 882 A1, which describes a seabed flowline connected to a conventional tanker serving as a floating storage facility by a system comprising a three-leg mooring and a flexible riser. The mooring comprises anchors connected by anchor lines to a common node, and a mooring pendant extending from the node to the tanker. The flexible riser comprises a flexible rubber hose extending from the seabed to the tanker, the hose having a top section secured along part of the mooring pendant, and an intermediate section provided with buoyancy members and restrained by a tether to maintain it clear of the anchor risers and the mooring node.
The prior art includes U.S. Pat. No. 7,287,936 B2, which describes a shallow water riser for extending beneath a sea and above a seabed between a connection at the seabed and a connection to a floating support, the shallow water riser having a wave form between the seabed connection and the floating support connection, which is shaped, is of such length and is positioned to include at least two riser wave parts in succession. Each of the two riser wave parts including a respective lower wave part toward the seabed, followed by a crest away from the seabed, one of the crests being between the two lower wave parts, at least one of the lower wave parts being positioned to be in contact with the seabed and the shallow water riser being of such length to enable such contacts and crests. This system utilizes a 2D wave configuration.
The prior art also includes US 2011/0155383 A1, discloses a transfer system for transferring hydrocarbons, power or electrical/optical signals to/from the seabed to the vessel or other buoyant structure in the shallow water when exposed to the environmental loadings from wind, wave and current. The conduit transfer system comprising a flexible pipe or umbilical extending from the buoyant unit at one end and to the seabed at the other end; and a riser support fixed to the seabed for supporting the flexible pipe characterized in that the flexible pipe a plurality of buoyancy beads for creating one or more inverse catenary curves of the flexible pipe to provide an excursion envelope. The system utilizes a 2D wave configuration.
The prior art also includes US2004/0163817 A1, describes a riser system that compensates for the motions of an associated floating platform comprises a vertical pipe section supported by the floating vessel and extending downward from the vessel substantially perpendicular to the sea floor, and a horizontal pipe section connected to the associated sub-sea well equipment and extending away from the equipment substantially parallel to the sea floor. An angled elbow pipe connects the horizontal pipe to the vertical pipe. At least one of the horizontal and the vertical pipes incorporates a flexing portion comprising a plurality of recurvate sections of pipe connected end-to-end with alternating curvatures. In one embodiment, the central axis of the flexing portion lies in a single plane and takes a sinusoid path. In another embodiment, the central axis of the flexing portion takes a three dimensional helical path. This system is not suitable for a flexible riser system.
It is therefore a need for a configuration for a flexible riser or a cable in which the riser or cable is not subject to excessive bending, overstretching and interference with mooring or buoyancy devices, particularly in shallow water.